Next generation mobile communication systems are standardized aiming at efficiently connecting to wired and wireless communication networks and providing integrated services, rather than being limited to providing simple radio communication services like previous generation mobile communication systems. Along with such demands for high-speed high-capacity communication systems capable of processing and transmitting a variety of information such as images, wireless data, and the like, there is required the development of techniques for transmitting a large volume of data close to the capacity of a wired communication network through a wireless communication network, getting out of voice-oriented services.
In an Automatic Repeat reQuest (ARQ) scheme, when a receiver successfully receives data, it transmits an acknowledgement (ACK) to a transmitter so that the transmitter may transmit new data. Contrarily, when the receiver does not normally receive data, the receiver transmits a not-acknowledgement (NACK) to the transmitter so that the transmitter may retransmit the data.
Recently, a Hybrid-ARQ (HARQ) scheme, which is a technique combining an existing ARQ scheme with channel coding of a physical layer, is proposed to improve transmission efficiency when data is processed. In the HARQ scheme, since data transmission errors are processed in the physical layer, the errors can be further promptly corrected as compared with the ARQ scheme in which the errors are processed in a higher layer. Therefore, the HARQ scheme is suitable for high-speed data transmission, and for this reason, the HARQ scheme is an important technique adopted in fourth generation communication systems.
Downlink means a communication from a base station to a user equipment, and uplink means a communication from the user equipment to the base station. In the downlink, a transmitter may be a part of the base station, and a receiver may be a part of the user equipment. In the uplink, a transmitter may be a part of the user equipment, and a receiver may be a part of the base station. Hereinafter, data transmitted in a HARQ scheme is referred to as HARQ data, and data transmitted in an ARQ scheme is referred to as ARQ data.
HARQ is performed by at least one HARQ entity existing in the user equipment or the base station. The HARQ entity allows transmission of data to be continuously processed while waiting for a feedback (an ACK or NACK signal) for a successful or unsuccessful reception with respect to transmission of immediately previous data.
In uplink transmission, if the user equipment receives resource assignment from the base station and indicates associated HARQ information to the HARQ entity, the HARQ entity performs a HARQ process indicated by the HARQ information. In the user equipment, a plurality of parallel HARQ processes may be used to support the HARQ entity.
The transmitter transmits HARQ data to the receiver, and the receiver transmits an ACK/NACK signal to the transmitter. If the transmitter receives an ACK signal from the receiver, the transmitter transmits new HARQ data. Contrarily, if the transmitter receives a NACK signal from the receiver, the transmitter retransmits the previously transmitted HARQ data. If the transmitter receives a NACK signal again, the transmitter retransmits the previously transmitted HARQ data as many times as a maximum number of retransmission.
Due to degradation in channel conditions, an error may occur in the ACK/NACK signal such that the transmitter receives a NACK signal although the receiver transmits an ACK signal (this is called as an ACK-TO-NACK error), or contrarily, the transmitter receives an ACK signal although the receiver transmits a NACK signal (this is called as a NACK-TO-ACK error). In the case of the ACK-TO-NACK error, since the transmitter retransmits the data although the receiver has successfully received the data, only the data is duplicated, but not lost. However, in the case of the NACK-TO-ACK error, since the transmitter transmits new data without retransmitting the previous data although the receiver has failed to receive the previous data, there is a problem of losing the data.
Therefore, required is a method of transmitting data using HARQ, which can reduce loss of data invited by an error in the ACK/NACK signal.